Firing Plates

I have been making plates and other serving bowls and they are warping in the glaze kiln. I bisque fire at 06 and glaze at 6. I am using clay and glazes rated for that temp. Why are my pieces doing this? Thx!

What clay body are you using?Lots of possible reasons:
1. Kiln is overfiring slightly. Use witness cones to confirm the actual heat work.
2. Clay is not perfectly formulated for cone 6. Try cone 5.
3. Pots are uneven. This is especially important for bowls.
4. Pots are drying unevenly.
5. Pots are deforming when removed from the wheel, and going back to that shape during firing. Clay has a memory.
6. Kiln is cooling too quickly.

All clay bodies are subject to warping if the pot is not well made. This is especially true for porcelain and some white stoneware bodies.

What clay body are you using?Lots of possible reasons:1. Kiln is overfiring slightly. Use witness cones to confirm the actual heat work.2. Clay is not perfectly formulated for cone 6. Try cone 5.3. Pots are uneven. This is especially important for bowls.4. Pots are drying unevenly.5. Pots are deforming when removed from the wheel, and going back to that shape during firing. Clay has a memory.6. Kiln is cooling too quickly.

All clay bodies are subject to warping if the pot is not well made. This is especially true for porcelain and some white stoneware bodies.

Thanks for all the info.I don't have any problems with any other pieces so I was wondering if the kiln was cooling to quickly. These particular plates and bowls were slabs, I used a slab roller so the thickness seems even. They could have dried uneven but I did dry them for several weeks and were not warped in the bisque firing. I don't have this issue with my thrown pieces. Overfiring could be an issue and I will use a witness cones on my next firing.

This multitude of forming tecninques will take a clay body that can withstand this slabing- press molding and then fired on a thrown rim.
I am not a cone 6 guy so others will chine in on bodies in that remp range that can withstand this-I know of many in hight temp ranges but these are no good for your cone 6.
Which clay body are you doing this with?
Mark

Are you changing directions as you roll the slabs?
Are you lifting them somehow that they are not always flat?
What type of clay body are you using?
Are you attaching lips or impressing the form on a frame of some sort?
Can you post a photo?

It is not unusual for slabs to warp, particularly if they are made with a slab roller. Make sure you gradually flatten them with the roller, flipping and turning the slab for each rolling. Even so, warping can be an issue. I have had much better luck rolling slabs by hand, but I was fortunate to have someone teach me how to do that. It is difficult to move slabs without distorting them. I try to keep them on canvas or some other fabric while moving them. You can also try putting a sheet of very thin plastic as a skin on a slab to stabilize it while moving it. I have done that, leaving the plastic on the slab while I shaped it to a mold and then removing the plastic for the clay to dry.

I have not yet made plate and bowls like you are doing, but I would like to try it. I wonder if it would help to use a sponge or rubber rib to work the slab a bit after it was on the plaster bat on the wheel and before throwing the foot. Maybe that would help align the clay particles, especially so that they match the foot ring.

I actually like the distortion that large slab made bowls can have, but I know you don't want that with plates and some bowls.

clay has a memory. that memory comes out in drying and firing so if all your pieces are warping, try handling and forming them a different way and see if it helps.

Slabs are kind of interesting when it comes to warping, the clay memory as you push the clay into a slab sets its up to wanting to pull back to an unstressed condition, which is why when you roll a slab you always want to pull it free from your canvas and in that state try to work the surface with a rib or something else to try to give the clay that unstressed memory to resist warping. Also, one thing you can do to make your plates more stable for stacking and tables is to not make a foot that is continuous but to make a 3 point foot so even if the plate/bowl warps ... it warps around those 3 points and will always be stable ... simple but easy fix.

As Diane said, I think the factor here is the slab roller. On a wheel, you gradually shape a piece and the clay molecules have a chance to align. Slab rollers stretch the clay quickly in one direction in a single pass. The clay molecules are forced, in one go, into shape.

I would try rolling in different directions by hand first - before putting it through the slab roller. I would even give them quick pass again by hand when they come out. That is the fastest way I can think of to create the right memory. Other than that, you could do several passes on the slab roller, gradually decreasing thickness while flipping and turning the slabs. To me this would be a time drain because you would have to re-adjust the slab roller constantly.

You folks are wonderful. I was introduced to the world of clay many...many years ago and but put it aside when kids, family and job issues took root in my life. With a great support system I resurrected my passion a few years ago and have a wonderful studio in my house. I sort of shyed away from joining The Ceramic Community as I am a hobbyist thou a serious one. This is my first post and I am kicking myself for not joining sooner. Most of the things you all have mentioned I knew at one time but not having interaction with other "mud" minded people I sort of second guess myself and forget the life of the clay. I'll try and post some pics and show you my issue. I do like them actually but sometimes you just need a flat plate! Thanks all!

With the right clay body you can do many things-for me in High fire cone 10 whiteware for slab work that requires serious slab-then press work and hold up thru all that I use a body made by Laguna called 1/2 & 1/2
Its 1/2 Dave's Porcelain and 1/2 WSO-This body stays flat and you can torture it with many forming options-I do not throw this body only slab roll it . This body has some tooth in it (as you can get it with sand or grog)

In mid range you should look for a clay body that will take this stress and remain flat- you can ask your supplier about it. You can also mix your own by wedging two bodies and testing it first.
Mark

I have been making plates and other serving bowls and they are warping in the glaze kiln. I bisque fire at 06 and glaze at 6. I am using clay and glazes rated for that temp. Why are my pieces doing this? Thx!

I use the methods you describe, but as ever, the devil is in the details. As others have pointed out, slab rollers stretch the clay in one dimension only, so it is best for round plates to hand roll, flipping and turning to roll the slab in multiple directions. I use paint stirring paddles (get some of the big long ones the pros use to stir 5-gallon buckets as well as the shorter, thinner 1-gallon stirring sticks) as spacers on either side of the slab so that at the end the rolling pin rolls along the spacer sticks and the clay will be exactly that thick through the entire slab. For plates and other flatware that I do not want any warpage, I roll the slab out using two canvas covered plywood work boards. Roll it once on the first board; lay the other board on top, canvas side down so the developing slab is sandwiched between the two boards; flip the sandwich; take board #1, which is now on top, off; rotate the board a bit, put the spacers back on the board, and roll it for the second roll; and so on until you have rolled it down to the spacer sticks. I find that just picking the slab up with your fingers can cause unseen stresses at the pinch points that leads to warpage. (More on this in a moment). When the slab is finished rolling, I stroke the top side gently with a rubber rib to smooth the surface of any minor irregularities and then place the plate mold upside down in the center of the slab. Then I slide my hand under the plywood work board and quickly but gently flip this sandwich so the slab flops around the plate mold. So far, the slab has never been actually picked up off of any of the alternating work surfaces, always sandwiched and flipped. Now that the slab is on the plate mold, rough-trim the gross excess and move to the wheel. Get the plate mold mounted on the bat pins and trim/finish the back side of the slab as you do now and add the foot ring coil, etc. Let it set up on the plate mold until firm, and then place a ware board on top of it, and flip this sandwich one last time to turn the leatherhard plate onto the ware board for final drying. Still, the plate has never been picked up by pinched fingers.

Off the topic of these plates but on the related topic of moving large slabs of clay without picking the slab up with pinched fingers: Others have mentioned smoothing a piece of thin plastic film onto the finished slab to support it while moving it. Another easy way is to use the same two paint sticks. Gently lift one edge of the slab and slip one stick underneath. Lay the other stick on top of the slab, aligned exactly with the first stick underneath. Now pinch the two sticks together keeping your hands towards the ends of the sticks, and lift the slab so that it hangs from between the sticks. Carry it wherever you are going next and lay it down for the next step. The sticks distribute the pinching pressure of your fingers over the entire edge of the slab rather than leaving pinch-prints from your finger/thumb. Extremely large slabs can be carried this way without the weight of the slab tearing it around your fingers at the pinch point.

I am having trouble with throwing 9-10 inch pie plates. They look great but crack when they dry. Not all of them crack just some of them. Any ideas on what I am doing wrong or what I can do to fix this issue? I use cone 5-6 clay.

I am having trouble with throwing 9-10 inch pie plates. They look great but crack when they dry. Not all of them crack just some of them. Any ideas on what I am doing wrong or what I can do to fix this issue? I use cone 5-6 clay.

Thank you for any help.

It's quite likely that you need to compress the bottom more when you throw them, another possible cause is lack of freedom of movement as they are drying - i.e. they may be stuck to the batt/ware board and unable to contract freely, too much water when throwing can also cause these problems.

Debora, like ajay said, try compressing the bottoms more. I use a wood rib to compress the inside bottoms, of most my wares, and have my students do the same. I have seen a dramatic drop in s-cracks because of this.

Thank you everyone for your suggestions. I think maybe the lack of movement and the drying. I do compress using a rib several times, but I leave the plate on the bat to dry. My studio is usually a bit on the cool damp side. I will trim the wall/base area more and wire cut it from the bat right away, then cover it more and longer. It usually happens about a third of the time with larger wide surfaces, so I knew it was something I was doing wrong and not the clay or the firing. I'd rather have it crack before the firing. Reclaiming isn't as much of a dreaded thing as it used to be for me.